The layered composite material actuator is used for inhibiting the vibrations of airplanes, missiles and space aeronautic structures. Recently, study is widely carried out on the monolithic piezoelectric actuator which generates a high stress as a high performance actuator for controlling the vibrations of the piloting panel of airplanes and marine structures.
However, the monolithic piezoelectric actuator is low in its deformation rate to make it impossible to produce a large displacement, thereby rendering it unsatisfactory as an actuator. Further, the fatigue which cannot be avoided after repeated actuations is one of the problems to be solved. In an attempt to overcome these problems, there are devised and practically used the bimorph piezoelectric actuator (Bimorphs), the reduced and internally biased oxide wafer (RAINBOW), the THUNDER (thin layer unimorph driver) and the like.
The bimorph piezoelectric actuator is that in which the polarizing directions of two piezoelectric ceramics are oppositely arranged so as to produce a deflection based on the tension and compression. This bimorph piezoelectric actuator generates a considerably large displacement, but its actuation power is weak.
The reduced and internally biased oxide wafer (RAINBOW) is that in which one face of the piezoelectric ceramic (PZT) is made contracted through chemical reactions. If graphite is laid together the PZT, and if they are heated to 975° C., then oxygen atoms fly away from the boundary between the PZT and graphite, so that the metallic non-piezoelectric material layer and the piezoelectric material layer are integrated together.
If this structure is cooled, then a curvature is formed owing to the difference between the thermal expansion coefficients of the two layers. In this curved face piezoelectric actuator, a large displacement is produced through a pumping motion, but the ceramic dome structure is vulnerable to impacts.
The THUNDER™ (thin layer unimorph driver) which was developed by the NASA of the United States is a ceramic actuator which produces a large displacement even with a low input voltage, and consists of a metallic material and a piezoelectric thin layer stacked together. A high temperature thermosetting adhesive is spread on stainless steel, and then, a piezoelectric ceramic is laid thereupon. Then aluminum foil is covered thereon, then a curing is carried out at 325° C., and then, cooling is carried out down to the normal temperature, thereby completing its manufacture.
A curvature is formed owing to the differences between the thermal expansion coefficients of the involved elements. Due to this geometrical characteristic, if an electric field is imposed on the ceramic layer, the deformation of the ceramic layer produces a curvature so as to cause a displacement of the central portion. The THUNDER™ shows the most superior performance among the unimorphs which have been developed so far. However, its overall specific gravity is approximately same as that of steel, that is, it is too weighty as a space-aeronautic component.
In this context, the present inventors devised that the metallic layer portions of the THUNDER™ are replaced with a fiber-reinforced polymer layer to lead to a lightweight, and the fiber-reinforced direction and the stacking sequence are optimized. Thus a curved shape actuator device consisting of an electro-active layer and fiber-reinforced composite layers, in which the actuation displacements are improved, has been developed.